This invention relates to fractionally crystallizing pure xylitol from an aqueous reaction medium in which the xylitol is formed by reduction of xylose.
In recent years, many attractive processes for making xylitol have been developed. These processes have involved extracting xylose from naturally occurring hemicellulosic plant materials and then reducing the xylose to xylitol.
Hemicellulosic materials such as corncobs, maize, cotton seed hulls and husks, sunflower hulls, oat hulls, peanut shells, rice hulls and husks, and bagasse, particularly oat hulls, have been found to be particularly interesting sources of xylose. Extraction of xylose from such hemicellulosic materials has been conveniently carried out by one or more hydrolysis steps, followed by separation and crystallization of a xylose of high purity. Reduction of the xylose, obtained thereby to xylitol has been suitably carried out in a conventional manner by a chemical or catalytic reduction in an aqueous medium.
However, a significant problem has remained, i.e., how to obtain a substantially pure xylitol from the aqueous medium in which reduction of xylose was carried out. As formed in the aqueous reaction mixture, the xylitol, amounting to about 30 weight percent of the mixture, has been mixed with small amounts of unreduced xylose as well as other residual contaminants from the hemicellulosic material, such as polysaccharides. Typically, the unreacted xylose, usually amounting to about 0.3 to 0.5 weight percent, has been the most serious contaminant. It has been found that adverse physiological effects are associated with ingestion of xylose by mammals, e.g., eye disorders. For this reason, when the xylitol is intended for human consumption, its xylose content must be negligible, i.e., no more than about 0.10 weight percent xylose.
Heretofore, xylitol had been isolated by purifying the aqueous reaction medium with ion exchange resins and then either evaporating off the solvent to leave xylitol as the residue or fractionally crystallizing the xylitol from an alcohol-water mixture. It has been found, however, that isolating xylitol by evaporation yields a product containing substantial amounts of unreduced xylose. On the other hand, when substantially pure xylitol has been isolated by fractional crystallization, an alcohol-water mixture has remained. This alcohol by-product has contained relatively large amounts of xylose and xylitol, as well as of alcohol, which has made it too valuable to be merely thrown away. Moreover, this alchohol by-product has represented a potentially useful source of nutrient material. However, expensive processing has been required in order to recover the alcohol and convert this by-product to a useful, i.e., alcohol-free, nutrient material. There has been an unfilled need therefore for a process for expeditiously obtaining substantially pure xylitol from the aqueous reaction mixture in which it is formed whereby a useful xylose-xylitol by-product is obtained which does not require further involved processing.